By Lucia Mutikani WASHINGTON (Reuters) – U.S. retail sales were unexpectedly flat in September as households cut back on purchases of motor vehicles and other big-ticket items like electronics and appliances amid stubbornly high inflation and rapidly rising interest rates. But consumers are not rolling over yet, with the report from the Commerce Department on Friday also showing a measure of underlying retail sales rising last month, thanks to strong wage gains and savings. These so-called core retail sales were also stronger than initially thought in August. “Consumer staying power may be wan…